The present invention relates to aircraft engines, more specifically to a nacelle, which can include an inlet area, a nose cowl area, and a fan cowl area for turbine engines, and a mitigation device to be fitted over a nacelle or other areas on an aircraft in order to reduce flow drag.
Flow drag not directly associated with the production of lift is referred to as parasitic drag, which is composed of drag of various aerodynamic components. Important to the current invention is skin friction drag, which is the drag on a body resulting from friction over its contact surfaces. There are mainly two cases where the flow in the boundary layer is entirely laminar or entirely turbulent over the surface. In a usual application the boundary layer is normally laminar near the leading edge of the object undergoing transition to a turbulent layer at some distance back along the surface.
For an aircraft engine nacelle, it has been calculated that the critical area of concern is the first one-third of the nacelle, or 30 inches back from the leading edge where a laminar boundary layer begins to develop at the leading edge. Downstream from the leading edge the laminar boundary becomes unstable and is unable to suppress disturbances imposed on it by surface roughness or fluctuations in the free stream. Every component or element that has direct contact with airflow, including rivets, joints, seams, and fasteners, has an aerodynamic friction and can product drag.
Skin gaps or irregular matching seam-surfaces and recesses often cause location transition from laminar to turbulent flow. One area of interest is gaps or joints, such as at the interface between the lipskin and nose cowl and at the interface between the nose cowl the fan cowl. Another area of concern is nacelle lipskins that have been damaged and/or repaired. This is a common occurrence and introduces new drag potential.
Benefits of laminar flow come in the form of reduced friction drag. Because nacelle friction drag has been approximated to account for 4-5% of the total friction drag of the aircraft, any reduction in nacelle friction drag causes a corresponding reduction in fuel consumption and aircraft operating costs. Thus, it is desirable to maintain laminar flow over as much of the nacelle surface as possible.
A number of designs exist seeking to reduce drag over a nacelle associated with irregular surfaces and joints or surface gaps. Some, such as is described in U.S. Pat. No. 2,873,931, utilize an air blowing system. Others, such as is described in U.S. Pat. No. 5,368,258, utilize a suction chamber within the nacelle. While certain of such existing designs may well benefit new engine structures, these solutions require the owner of an existing aircraft to remove nacelle parts and replace them with new complex systems, having multiple working components. While the future trend is to integrate the inlet lipskin into the nose cowl and fan cowl sections to avoid connections of separate nacelle sections, greater than 99% of all currently in-service aircraft having nacelles remain with the separate-section construction. Removing an existing nacelle and replacing it with a composite, one-piece nacelle is cost prohibitive. There is a need to provide a drag reduction device that is cost effective for existing aircraft with a design that is scalable and adaptable for multiple areas and the many sizes and shapes of aircraft nacelles and intakes.